1. Field of the Invention
The present invention relates to a connecting film, a joined structure, and a method for producing the joined structure. The present invention relates particularly to a connecting film capable of electrically and mechanically connecting circuit members such as IC chips and liquid crystal display panels (LCD panels) in liquid crystal displays (LCDs); a joined structure including the connecting film; and a method for producing the joined structure.
2. Description of the Related Art
Conventionally, as a means of connecting substrates (circuit members), connecting materials in the form of tapes (e.g., anisotropic conductive films (ACFs)) have been used, which are formed by coating a release film with a curable resin containing conductive particles dispersed therein.
The anisotropic conductive films (connecting films) are used for sticking together and electrically connecting together a variety of terminals, notably for connecting a flexible printed circuit (FPC) board or a terminal of an IC chip with an ITO (indium tin oxide) electrode formed on a glass substrate of an LCD panel.
Examples of connecting films include a connecting film obtained by dispersing conductive particles in an epoxy resin, and a connecting film obtained by dispersing conductive particles in an acrylic resin. These connecting films (organic resin layers) are cured by heating to connect substrates (circuit members).
The connected state of the substrates (circuit members) is generally grasped by measuring the cure extent of a connecting film (organic resin layer) with the use of an analyzer such as an IR or DSC. However, the measurement of the cure extent of the connecting film (organic resin layer) with the use of the analyzer presents a problem in which it is necessary to extract a measurement sample from a joined structure and set the measurement sample in the analyzer, which may take time and trouble.
As a countermeasure against the foregoing problem, there has conventionally been a method of utilizing such a property of a connecting film (organic resin layer) containing an imidazole-based curing agent that the connecting film turns red when cured, and viewing the color change of the connecting film to red as an indication of completion of curing (refer, for example, to Japanese Application Laid-Open (JP-A) No. 04-145180).
Nowadays, however, pressure bonding of connecting films at a lower temperature in a shorter period of time is demanded and thus acrylic connecting films containing radical curing agents are becoming frequently used in place of connecting films containing imidazole-based curing agents. Accordingly, device makers, in particular, are strongly demanding development of a technique for estimating the cure extent of connecting films (organic resin layers) by visual observation, in the case of the acrylic connecting films containing radical curing agents as well as in the case of the connecting films containing imidazole-based curing agents.
Here, connecting films have been disclosed whose color-developing components develop color as curing proceeds (refer, for example, to JP-A Nos. 11-307154 and 2007-91798). Regarding each of these connecting films, however, the color-developing component is in a color-developing state after the connecting film has been bonded to a substrate under pressure, which is problematic in that it is difficult to identify corroded portions in an external appearance inspection (e.g., inspection for the number of conductive particles, a deformed state, etc.) and a corrosion test. Also regarding each of these connecting films, the color-developing component does not develop color before the connecting film is bonded to the substrate under pressure, thereby causing a problem in that when the connecting film is set in a temporarily affixing device, it is hard for a recognition sensor to recognize the connecting film. Meanwhile, an adhesive containing a color-erasing component (salt of a combination of a cyanine pigment and an organic boron compound) which is colored when applied but becomes colorless by photoirradiation after applied has been disclosed (refer, for example, to JP-A No. 05-132655); however, the color-erasing reaction in JP-A No. 05-132655 does not stem from reaction of a curing agent, unlike the present invention. Therefore, when the adhesive of JP-A No. 05-132655 is used for a connecting film without any change made to it, color erasure that represents the cure extent does not take place, and places subject to color erasure are not necessarily where curing has progressed, thereby causing unevenness of color erasure (JP-A No. 05-132655 is merely an invention for allowing the applied state of the adhesive to be recognized by color erasure).